The present invention relates to the joining of fabrics made of advanced fibers for making pre-forms for composite materials.
More particularly, the invention provides a sewing mechanism which is useful for joining fabrics with a brittle thread.
In the present specification, as in other prior documents, the terms “severing thread” “brittle fiber thread” “advanced fiber thread” “high performance threads” “technical fiber” are used interchangeably to designate a thread made from glass, ceramic, graphite or carbon fibers of a commercially or experimentally available type. No novelty is claimed here regarding the thread.
Optimum stitching results using the said mechanism according to the present invention are obtained when the sewing needle, which is described in our co-pending application which is hereby incorporated herein by reference, is used.
Composite fiber-reinforced materials are widely used wherever a high strength/weight ratio is needed, typically in aerospace vehicles, sports equipment, and to a lesser extent in land and sea vehicles and their equipment. Glass and carbon fibers have long been used for reinforcing other materials called matrices, particularly polyester, epoxy and other thermosetic and thermoplastic polymers, in order to obtain high performance composite materials. However the joining of layers of reinforcing fibrous materials can be difficult, as joining methods applicable to textile materials are generally inapplicable to high performance fabrics and yarns. Reliable joining techniques are needed also for these advanced materials in order to carry out tasks such as producing a multi-layer or lamina pre-form which subsequently is impregnated with the resin that turns to be the matrix.
High strength threads are preferred for sewing together sheets of high performance fibrous layers. Threads based on carbon or glass fibers are commercially available. It has however been found that when sharply bent or when friction is applied, such yarns tend to break. The standard sewing machines apply very sharp bends to the threads as well as friction with the sewing needle eye during the sewing process hens they are inadequate for stitching with brittle yarns.
The state of the art can be assessed by a review of the following recent US Patents.
Leska Sr. discloses a chain stitching apparatus in U.S. Pat. No. 4,757,775 for reinforcing or attaching composite materials. This design is intended to keep all critical parts on the upper side of the material. No mention is made about prevention of brittle thread breakage.
A further stitching head proposed by Cahuzak is disclosed in U.S. Pat. No. 5,515,798, also without means for preventing the sharp bends liable to break a composite thread. The same inventor also details a method for reinforcing a sheet for a composite component in U.S. Pat. No. 5,759,321.
In U.S. Pat. No. 5,333,562 LeMaire et al. propose a method of stitching a fabric using a composite thread to produce a fiber-matrix piece. The needle thread is flexible, while the shuttle thread is composite.
Baxter discloses a blind stitching apparatus featuring a curved needle seen in U.S. Pat. No. 5,829,373. The stitches are placed at discrete locations to join pre-forms to make a composite component.
A sewing machine with both needle and bar rocking and thread tension releasing mechanisms actuated by a single actuator is disclosed by Hori in U.S. Pat. No. 6,055,920.
Sakuma in U.S. Pat. No. 6,101,961 proposes a rather complex mechanism for thread feeding intended for high speed sewing. Upstream grippers grip both upper and lower looper threads and close during stitch forming while opening when the cloth is moved. Downstream grippers also grip the threads but open during stitch forming and close when the cloth is moved.
In U.S. Pat. No. 6,145,457 Imaeda et al. disclose a sewing machine which includes mechanisms for adjusting and releasing thread tension. By driving a cam rod holding several cans, a single motor is used to drive the various mechanisms.
Yamazaki in U.S. Pat. No. 6,595,150 B2 describes a pneumatically-operated thread tension controller including a solenoid valve, a microcomputer, an air-pressure regulator and a pneumatic cylinder driving thread tension disks.
Johnson et al. in U.S. Pat. No. 6,645,333 disclose a method for inserting z-axis reinforcing fibers into a composite laminate. After each insertion the feed thread is severed and there is thus no need to sharply bend the thread. However the strength of the joint using many short threads is clearly inferior to stitching with a continuous thread.
In U.S. Pat. No. 6,729,251 B2 Nishikawa et al. disclose a thread regulating device relating to a multi-needle chain stitch machine.
While some of the mechanisms seen in the prior art could be used in a machine intended for composite threads, there is not seen a machine that fully relates to the requirements of stitching with a composite thread while avoiding small bend radii and preventing friction of the sewing thread in the sewing needle as a result of thread being pulled through the needle while stitching is in progress.
Furthermore the mechanisms seen are both above and below the fabric being stitched. The idea of eliminating the thread mechanism below fabric level is seen only in the Leska patent which makes no provision for brittle threads.